1. Field of the Invention
The present invention relates to an apparatus for measuring various characteristics of an optical fiber. More particularly, the present invention relates to an apparatus for measuring the characteristics of an optical fiber by inputting an optical pulse to the optical fiber, by multiplexing returned light from the optical fiber and locally oscillated light and by performing heterodyne detection.
This application is based on Japanese patent application No. Hei 9-174767 the contents of which are incorporated herein by reference.
2. Description of Related Art
FIG. 2 shows a configuration of an apparatus for measuring the characteristics of an optical fiber according to a related technique. A frequency control circuit 1 controls a driving circuit 2 to stabilize the frequency (frequency f1) of coherent light 3a generated by a light source 3. When an optical pulse generation circuit 4 converts the coherent light 3a into pulse light 4a, the pulse light 4a sequentially passes through an optical directional coupler 5 and an optical connector 6 and reaches to an optical fiber 7 to be measured. Backward scattering light and reflected light such as Rayleigh scattered light and Brillouin scattered light are generated depending on the state of the optical fiber 7 to be measured, and returned light 6a as a part of such light sequentially passes through the optical connector 6 and optical directional coupler 5 to be input to an optical directional coupler 15.
Meanwhile, a frequency control circuit 11 controls a driving circuit 12 to stabilize the frequency (frequency f2) of coherent light 13a generated by a light source 13. The coherent light 13a is locally oscillated light as described above. The optical directional coupler 15 multiplexes the returned light 6a and the coherent light 13a to output an optical signal 15a to an optical detection section 16. The optical detection section 16 performs coherent detection of the optical light 15a to convert it into an electric signal 16a. A signal processing section 17 calculates various characteristics of the optical fiber 7 to be measured based on the electric signal 16a to generate characteristic distribution that depends on positions on the optical fiber 7 to be measured.
In order to detect the returned light 6a properly in the apparatus for measuring the characteristics of optical fiber shown in FIG. 2, a relative frequency difference .vertline.f1-f2.vertline. between the coherent light 3a and the coherent light 13a must be properly set in accordance with the frequency components of the returned light 6a. For this purpose, according to the related technique, the frequency of each of the light sources 3 and 13 is controlled to determine the relative frequency difference .vertline.f1-f2.vertline.. However, it is not easy to set the relative frequency difference .vertline.f1-f2.vertline. accurately by controlling the frequencies of the two light sources independently. As a result, the accuracy of the detection of the returned light 6a is adversely affected by an error included in the relative frequency difference .vertline.f1-f2.vertline..